The use of ultraviolet (UV) radiation for phototherapy is well established. In fact, UV therapy is now involved in the treatment of more than 40 types of skin diseases and disorders such as psoriasis, vitiligo and eczema. Phototherapy studies of UVB wavelengths between 260 nm and 320 nm found that a narrow-band UVB emission centered at approximately 312 nm is most effective for phototherapy while at the same time limiting undesirable erythemal effects. Since the skin's erythemal (or sunburning sensitivity) is at its maximum at about 297 nm, a narrow-band emission at about 312 nm allows a patient to have longer treatment times before an erythemal response appears.
The Gd3+ 6P7/2→8S transitions are ideal for 312 nm narrow-band emissions. However, f-f transitions of rare earths, being parity forbidden, are very weak and the use of a sensitizer is necessary to obtain a useful emission intensity. One of the first narrow-band UVB phosphors to be developed was sensitized with bismuth, e.g., (Gd0.5,La0.487)B3O6:Bi0.013. On excitation by 254 nm radiation, this borate phosphor emits the characteristic radiation with a very narrow band centered on 312 nm. However, because of the toxicity of the bismuth sensitizer, other narrow-band UVB phosphors were developed, in particular (Gd0.45,Y0.5)MgB5O10:Ce0.05, which is described in U.S. Pat. No. 4,319,161.
The method of making the Ce-sensitized phosphor described in U.S. Pat. No. 4,319,161 involves dry mixing oxides of Gd, Y, Ce, Mg and boric acid, and then subjecting the mixture to three firings in a weakly reducing atmosphere. Later in U.S. Pat. No. 6,007,741, Hunt et al. proposed an improved process to prepare the (Gd0.45,Y0.5)MgB5O10:Ce0.05 phosphor by milling the reactants in a saturated aqueous solution of magnesia and boric acid prior to firing three times in a slightly reducing atmosphere. The later method was developed to increase the homogeneity of the fired phosphor cake and reduce the tendency of the fired cake to stick to the firing boats. However, three firing steps were still needed.